Commercial polypropylene impact copolymers, industrially referred to as heterophasic copolymers, block copolymers, or simply as “impact copolymers” (ICPs), are typically made in at least two stages series reactors, and typically are “propylene-based” meaning that propylene-derived comonomers or monomers predominate in the ICP. Most ICP's comprise a homopolymer component made in the lead reactor and a copolymer component made in the second reactor to form an intimate polymer blend. The propylene monomer may be polymerized in slurry or gas phase process in the lead reactor. After the polymerization in the lead reactor, the porous polypropylene granules are stripped of the propylene monomer and transferred into a gas phase reactor where the propylene and ethylene comonomers are fed into the reactor to incorporate ethylene-propylene copolymer within the pores of the granules under fluidized conditions.
Because of the limitations of commercially available catalyst systems presently available to incorporate target amount of low ethylene content ethylene-propylene copolymers in the homopolymer granules, the physical properties that can be commercially achieved are also limited, in particular, in the Izod impact strength and elongation at break, etc.
This disclosure addresses the means to reliably and commercially manufacture propylene-based heterophasic copolymers that exhibit improved elongation at break, ductility, gloss and impact strength without compromising the stiffness (flexural modulus) of the resulting heterophasic copolymer by control of the copolymer content of the ICP, namely, at copolymer contents of greater than 10 or 15 or 20 wt % in the ICP. Also, the inventive ICP also improves the processability (or “flowability”) of ICP granules in the transfer system from the polymerization stage to the finishing area to maintain the preferred production rate.
Related patents and publications include U.S. Pat. Nos. 8,207,272; 8,067,510; 8,044,135; 7,977,435; 7,935,766; 7,872,074; 7,772,324; 7,348,381; 7,309,742; 7,449,522; 6,660,808; 6,399,707; 6,384,142; 6,087,459; 5,258,464; US 2001/034411; US 2004/092631; US 2010/105848; WO 2012/010678; WO 2001/58970; WO 03/044086; WO 99/20663; and WO 98/21275.